High-rate biological waste water treatment process using activated sludge recycle

In a biological waste water treatment process, a method for removing organic matter, phosphorus and nitrogen from influent waste water comprising the steps of:

• (a) introducing influent waste water to an upstream-most one of an anaerobic reactor in an anaerobic zone which includes multiple serially fluid-connected one of said anaerobic reactors;
• (b) introducing effluent from a downstream-most one of said anaerobic reactors in said anaerobic zone to an upstream-most one of an anoxic reactor which is serially fluid-connected to other downstream anoxic reactors;
• (c) introducing the effluent from a downstream-most one of said anoxic rectors in said anoxic zone to a downstream aerobic zone which includes multiple serially fluid-connected aerobic reactors in which dissolved oxygen is brought into contact with said anoxic zone effluent to produce a nitrified mixed liquor and wherein an upstream one of said anoxic reactors has a hydraulic residence time which is substantially equal to the hydraulic residence time of another downstream one of said anoxic reactors;
• (d) separating the effluent from said aerobic zone into a layer of activated sludge and a clarified supernatant having reduced levels of nitrogen and phosphorus;
• (e) recycling a portion of said activated sludge to the process stream entering said upstream-most one of said anoxic reactors in said anoxic zone;
• (f) removing a portion of said activated sludge as waste;
• (g) recycling to said upstream-most one of said anaerobic reactors a portion of said anoxic zone effluent from at least said downstream-most one of said anoxic reactors;
• and
• (h) recycling to said upstream-most one of said anoxic reactors said nitrified mixed liquor produced in said aerobic zone.